The present invention relates generally to hydraulic drive systems of the type used for advancing and retracting a ram for controlling positioning of a workpiece or the like, and is particularly concerned with a hydraulic drive assembly for positioning the electrode of an electrical discharge machine.
In a typical hydraulic drive system, a supply of hydraulic fluid is provided from a reservoir to a pump inlet. The pump outlet is connected via input lines to opposite ends of a hydraulic cylinder, with control valves for selectively cutting off the supply of fluid to respective ends of the cylinder. Return lines are provided from each end of the cylinder to the reservoir, with control valves for controlling the connection of the respective ends of the cylinder to the reservoir. A piston in the cylinder moves back and forth in response to the supply of pressurized fluid to opposite ends of the cylinder, and is connected to a ram extending out of the cylinder for positioning a workpiece, machine part, or the like. This is a four way system including four connecting lines and corresponding valves between the pump, cylinder and reservoir, and a connecting line from the reservoir to the pump. Such systems are typically bulky, expensive, and have relatively slow response times.
Hydraulic drive systems of this type have been used in the past for controlling the positioning of an electrode of an electrical discharge machine, for example, although they were not ideal for this purpose in view of their size, expense, and relatively slow operation.
In a typical electrical discharge machine, a power supply or control circuit applies a pulsed waveform between an electrode immersed in a bath of liquid dielectric and a workpiece positioned below the electrode. When the gap between the electrode and workpiece is at an optimum distance, a spark traverses the gap and material is removed from the workpiece. The spark will occur again on the next voltage pulse if the optimum gap is maintained. A servo system is used to maintain the desired gap spacing. The voltage across the gap is detected, and is compared to the optimum voltage corresponding to the desired gap spacing. If the gap voltage is too small, the servo mechanism moves the electrode up. Conversely, if the detected gap voltage is too large, the electrode is moved down. The servo mechanism used is either of the lead screw ram type or a hydraulic system. These have the disadvantage of being relatively slow, and are generally large and expensive machines. A typical hydraulic system will be relatively slow to respond to a control signal to drive the electrode up or down, and is also prohibitively large and expensive for all but very large machine shops.